This invention relates generally to the field of disc drive data storage devices, and more particularly, but not by way of limitation, to an apparatus and method for reducing the generation of acoustic noise during initial stages of a seek operation wherein a head is moved from an initial track to a destination track.
A disc drive is a data storage device used to store and retrieve computerized data in a fast and efficient manner. A typical disc drive comprises one or more rigid discs that are coated with a magnetizable medium and mounted on the hub of a spindle motor for rotation at a constant high speed. The data are stored on the discs in a plurality of concentric circular tracks by an array of transducers (xe2x80x9cheadsxe2x80x9d) mounted to a radial actuator for movement of the heads relative to the discs. The heads are supported over the corresponding disc surfaces by air bearings set up by the rotation of the discs and are controllably moved by an actuator motor, such as a voice coil motor (VCM).
A problem that has received a great deal of attention in the disc drive industry is the generation of undesired acoustic noise as the heads are moved from one track to another (referred to as a xe2x80x9cseek operationxe2x80x9d). To maximize data transfer performance, efforts are made to minimize the time required for a seek so that the disc drive can proceed to transfer data to or from the destination track. The application of large amounts of current to the actuator motor to initially accelerate and then decelerate the head results in fast head movement to the destination track, but can also undesirably set up vibrations in the disc drive housing which can be annoying to a computer user, as well as detrimental to the operation of adjacent drives in multi-drive applications.
Accordingly, efforts have been made in the prior art to reduce the rate at which current is initially applied to an actuator motor to reduce the generation of unwanted acoustic noise. For example, U.S. Pat. No. 5,475,545 issued to Hampshire et al. and assigned to the assignee of the present invention, discloses a servo circuit which is used to control the position of heads of a disc drive using a modified velocity-controlled approach.
In a standard velocity-controlled seek, a velocity profile, indicative of a desired velocity trajectory for the head, is selected based on the distance between the initial and destination tracks. The head is first accelerated to a maximum velocity through the application of a large magnitude of current to the coil. The head then is maintained at this maximum velocity until the head reaches a predetermined distance from the destination track, at which point a large amount of current of opposite polarity is applied to the coil to decelerate and settle the head onto the destination track. Adjustments in the velocity of the head are repetitively made during the seek in relation to the difference between the actual velocity and the velocity profile. The desired velocity at any given point during the seek is referred to as a demand velocity, with the velocity profile comprising a series of demand velocity values.
At the beginning of the seek, the actual velocity of the head is substantially zero, whereas the velocity profile requires a demand velocity of substantially maximum velocity. This results in a large velocity error, and the servo circuit responds by applying the maximum available current to the coil, resulting in quick acceleration, but also large amounts of vibration. Accordingly, the Hampshire U.S. Pat. No. 5,475,545 reference modifies initial portions of the velocity profile by providing an initial demand velocity of zero and then incrementing the demand velocity during each of a succession of time periods by a fraction value until the maximum demand velocity is reached. In this way, the velocity errors at initial stages of the seek are reduced and the current is more gradually applied to the VCM until the maximum velocity is reached. By selecting an appropriate fraction value, the seek can still be carried out in a short amount of time, but with a substantial reduction in acoustic noise.
U.S. Pat. No. 5,657,179 issued to McKenzie et al. and assigned to the assignee of the present invention, provides an improvement over the Hampshire U.S. Pat. No. 5,475,545 reference by using a velocity demand fraction value that is varied during the initial portions of the seek, thus allowing further tailoring of the resulting current that is applied to the VCM. The use of a variable demand fraction value allows additional tailoring of the current to obtain optimal results.
U.S. Pat. No. 4,965,501, issued to Hashimoto, discloses a different approach to reducing noise at initial stages of a seek. Instead of reducing the initial demand velocities as taught by the aforementioned references, the Hashimoto U.S. Pat. No. 4,965,501 initiates a seek by determining a large velocity error and generating a large initial current command (as with the standard velocity-controlled approach), but then sets an output servo gain block to a very low level and successively increments the gain to gradually apply more current to the VCM until the maximum velocity is reached.
It will be noted that a tradeoff must be made between performing the seek in as quickly a manner as possible and reducing the generation of acoustic noise during the seek. The foregoing references provide different methodologies to tailor the initial application of current to arrive at an acceptable compromise, and have been found operative in the art.
Nevertheless, there are drawbacks associated with these and other prior art approaches. For example, adjusting the demand velocity as taught by the Hampshire and McKenzie references adds additional complexity to the servo circuit design (usually programming carried out by a servo processor, such as a digital signal processor). With continued increases in track densities, ever greater demands are placed on the ability of servo processors to provide robust servo performance; thus, the additional processing overhead required to implement the techniques of the Hampshire and McKenzie can be difficult to accommodate.
Likewise, while Hashimoto presents an operative solution, continually adjusting the output servo gain can also require additional processing steps, especially when the optimum servo gain for a particular head can vary at different locations across the disc. Hence, it can also be difficult to specifically vary the servo gain to effect initial stages of a seek and then ensure that the final gain is appropriate for operation once the head reaches the destination track.
Accordingly, there is a continued need for improvements in the art whereby seeks can be carried out in a manner which reduces the generation of acoustic noise, without requiring complex operations or the adjustment of various servo parameters, such as the velocity profile or the servo gain. It is to such improvements that the present invention is directed.
The present invention is directed to an apparatus and method for reducing the generation of undesired acoustic noise during a disc drive seek operation.
In accordance with preferred embodiments, a disc drive includes a rotatable disc with a recording surface on which a plurality of tracks are defined and an actuator which supports a head adjacent the tracks. The head is moved through application of current to a coil of a voice coil motor (VCM).
A seek is carried out to move the head from an initial track to a destination track on a corresponding disc surface using a servo circuit which outputs a series of current command values indicative of successive magnitudes of current to be applied to the VCM. A velocity profile provides a sequence of demand velocities indicative of the desired velocity at each of a succession of time periods during the seek. Closed loop velocity control is achieved by comparing the actual velocity of the head to the corresponding demand velocity and adjusting the current in relation to the velocity error therebetween.
During initial stages of the seek, however, open loop control is initially performed using a current profile table which shapes the initial stages of current to a desired waveform, irrespective of the velocity profile and the velocity error. The waveform is selected to reduce excitation of mechanical resonances in the disc drive and is applied to the coil regardless of the location of the initial track.